The present invention relates to an in-line type electron gun for a color picture tube, and more particularly to the structure of a first grid and a second grid which constitute the electron gun.
A prior-art electron gun for a color picture tube has a structure as shown in FIGS. 1 and 2 by way of example. As illustrated in FIG. 1, the electron gun includes three cathodes 1A, 1B and 1C which are arrayed orthogonally to the axis of the tube and at equal intervals on a straight line, and a first grid 2, a second grid 3, a focusing electrode 4 and an anode 5 which are disposed at predetermined intervals in this order from the side of the cathodes 1A-1C toward a screen not shown and each of which has apertures aligned with beam paths corresponding to three electron beams emitted from the cathodes 1A-1C.
The cathodes 1A, 1B and 1C, the first grid 2 and the second grid 3 construct a so-called "triode portion." Usually, variable voltages of 0-200 V are applied to the cathodes 1A-1C, a voltage of 0 V is applied to the first grid 2, and a voltage of about 600 V is applied to the second grid 3, whereby the electron beams 6A, 6B and 6C are formed. Further, the focusing electrode 4 is supplied with a voltage with which the electron beams 6A-6C are focused to the optimum on the screen though not depicted in the figure, and the anode 5 is supplied with a high voltage equal to that of the screen.
In order to maintain the orthogonalities of the electrodes to the beam paths, the parallelism among the electrodes, and the coaxialities between the respectively corresponding apertures of the electrodes, the electron gun for the color picture tube constructed as stated above is assembled in such a way that three mandrels arranged on straight lines and held parallel to one another are respectively passed through the three apertures of the electrodes, and that spacers each having surfaces parallel to each other are inserted in the interspaces between the respectively adjacent electrodes.
In case of such assemblage, the first grid 2 and the second grid 3 have heretofore been set up in order to secure the mutual parallelism thereof as disclosed in, for example, the official gazette of Japanese Utility Model Publication No. 15242/1985. More specifically, as illustrated in FIG. 2, regarding the first grid 2, the peripheral parts 7a and 7c of respective outer apertures 2a and 2c opposing to the second grid 3 are protruded to the side of the second grid 3 more than the peripheral part 7b of a central aperture 2b, while regarding the second grid 3, the peripheral parts 8a and 8c of respective outer aperatures 3a and 3c opposing to the first grid 2 are protruded to the side of the first grid 2 more than the peripheral part 8b of a central aperture 3b. Thus, only the outer peripheral parts 7a and 8a, and 7c and 8c of the electrodes 2 and 3 come into contact with the spacers (not shown) which are used for setting the mutual interval between the first grid 2 and the second grid 3. Therefore, the mutual parallelism between the first grid 2 and the second grid 3 can be enhanced.
With the prior art, the intervals la and lc between the outer apertures of the first grid 2 and the second grid 3 become, in effect, smaller than the interval lb between the central apertures thereof.
In general, in a color picture tube, cathode cutoff voltages (namely, cathode voltages with which cathode currents become "0") E.sub.kco need to be equalized for three electron beams to the end of equalizing the cathode drive characteristics of the electron beams corresponding to red, green and blue. It is known that the relationship of the following equation holds between the cathode cutoff voltage E.sub.kco and the dimensions of the triode portion: ##EQU1## where A denotes a constant, D the diameter of each aperture of the first grid 2, S the spacing between each cathode and the corresponding aperture of the first grid 2, T.sub.1 the thickness of the vicinity (for example, 22a in FIG. 2) of the aperture of the first grid 2, l the interval between the corresponding apertures of the first grid 2 and the second grid 3, and E.sub.c2 the voltage of the second grid 3.
In the case of the prior art, since the intervals la and lc are smaller than the interval lb as shown in FIG. 2, the spacings Sa and Sc need to be made greater than the spacing Sb in accordance with the relationship of Eq. (1).
In the triode portion in which the individual dimensions l and S are unequal, however, differences develop in lens characteristics which are formed in the triode portion, and differences also develop in the divergent angles of the electron beams which are emitted from the triode portion. As a result, the angles of incidence of the electron beams on a main focusing lens become unequal, and the focusing conditions of the electron beams become different. That is, the optimum focusing voltages V.sub.f of the electron beams become unequal. Moreover, this tendency intensifies as beam currents I.sub.b increase.
It has been experimentally and calculatively revealed that, in a case where the interval l is small and where the spacing S is great, the divergent angle enlarges relative to a case where the interval l is great and where the spacing S is small, so the optimum focusing voltage V.sub.f of the electron beam rises.
In the prior art, accordingly, the voltage V.sub.f of each of the outer beams becomes higher than that of the central beam. In actuality, when the beam currents I.sub.b are changed as shown in FIG. 3, the optimum focusing voltage V.sub.f of the central electron beam 6B shown in FIG. 1 becomes a characteristic 20 indicated by a solid line, and that of each outer electron beam 6A or 6C becomes characteristic 21 indicated by a broken line.
In this manner, with the prior-art electron gun, when the beam currents I.sub.b are changed, the central electron beam 6B and the outer electron beam 6A or 6C exhibit the different variations of the optimum focusing voltages V.sub.f. The prior art has therefore involved the problem that, when either electron beam is set at the optimum focusing condition, the other electron beam deviates therefrom, so a vivid picture is not produced on the phosphor screen.